Field of the Invention
The invention relates to a catalyst for reducing the nitrogen oxide concentration in a flowing medium, such as waste gas or flue gas of a combustion plant. The invention also relates to a method for the production of such a catalyst.
Due to the proven environmentally damaging effect of nitrogen oxides in the waste gas of a combustion plant, a technology has developed with the goal of catalytically reducing the nitrogen oxides. A method that has gained acceptance is the process known as selective catalytic reduction (SCR method), whereby the nitrogen oxides are contacted together with a suitable reducing agent, usually ammonia NH.sub.3, with what is known as a deNO.sub.x catalyst and are catalytically converted into nitrogen and water, which are environmentally safe.
German Patent DE 24 58 888 C, corresponding to U.S. Pat. No. 4,085,193, discloses, among other things, deNO.sub.x catalysts that contain vanadium and molybdenum as well as titanium as the main constituent. It has been proven that a molybdenum-containing catalyst does not attain the catalytic activity of a tungsten-containing catalyst. At present, that disadvantage with respect to activity can only be compensated for by increasing the catalytic volume of the molybdenum-containing catalyst.
Furthermore, it has been shown that the "intimate mixture" of the components of the catalyst, which proved to be of particular significance in German Patent DE 24 58 888 C, corresponding to U.S. Pat. No. 4,085,193, causes the catalyst to become poisoned relative quickly through volatile heavy metals and heavy metal compounds contained in the waste gas to be denitrogenated. The "intimate mixture" by definition excludes chemically stabilized and/or pre-calcined titanium dioxide TiO.sub.2 as the starting material for catalysts because, according to U.S. Pat. No. 4,085,193, a sufficient level of catalytic activity cannot be attained, as explained in column 3, lines 59ff of U.S. Pat. No. 4,085,193.
U.S. Pat. No. 4,952,548 discloses a catalyst for reducing nitrogen oxide concentration, which catalyst contains titanium oxide as a first component, molybdenum oxide MoO.sub.3 and/or tungsten oxide WO.sub.3 as a second component, and vanadium oxide and/or vanadium sulfate as a third component. A lower limit specified for the content of the second component is 3 atom %. With a molecular weight of 144 g for MoO.sub.3 and 96 g for TiO.sub.2, the proportion of MoO.sub.3 for the lower limit is therefore approximately 5% by weight. At present, a content of approximately 10 to 12% by weight, calculated as MoO.sub.3, has proven to be the most suitable molybdenum proportion for such a catalyst. It has been proven that the catalytic activity of a titanium-molybdenum-vanadium catalyst having the atomic ratios given in U.S. Pat. No. 4,952,548 is still below the catalytic activity of a commercially available titanium-tungsten-vanadium catalyst.
German Published, Non-Prosecuted Patent Application DE 35 31 810 A1 discloses a catalyst material for reducing nitrogen oxides, which material is created in a grinding step of titanium oxide in a calcined anatase modification, with an admixture of vanadium oxide and possibly molybdenum oxide. By partially substituting phosphorous, the sum of the active components vanadium oxide and molybdenum oxide lies at a volume of less than 1 atom % with respect to titanium. The catalytic activity is attained in this case not merely by mixing the substances but through the use of a grinding process which gives rise to a catalytically effective compound that differs from a mixture alone.
Published European Patent Application 0 313 755 A2 also discloses a molybdenum and vanadium-containing catalyst based on titanium oxide for the removal of nitrogen oxides. The concentration of molybdenum and vanadium contained in the catalyst through impregnation with a dissolved molybdenum and vanadium compound, calculated as MoO.sub.3 and V.sub.2 O.sub.5, is 5 to 15% by weight and 0.1 to 3.0% by weight, respectively.
Furthermore, for the production of such Ti/Mo/V catalysts, Published European Patent Application 0 360 548 A1, corresponding to U.S. Pat. No. 5,166,122, discloses a process whereby ammonium molybdate and ammonium metavanadate are mixed with metatitanium acid in water to form a kneadable mass, this mass is granulated, dried, calcined at 550.degree. C. and ground to a powder. The powder which is thereby obtained is mixed with water to form a slurry, into which a metal support is lowered so as to be coated. This is followed by further calcination at 500.degree. C.
Additionally, German Published, Non-Prosecuted Patent Application DE 28 46 476 A1, corresponding to U.S. Pat. No. 4,207,209, discloses a process for the production of a titanium dioxide-containing sintered product for catalytic removal of pollutants, whereby titanium dioxide is directly mixed and kneaded with ammonium vanadate and ammonium molybdate to form an aqueous slurry. The resulting mixture is dried, pulverized and pre-calcined, whereby the pre-calcination serves to deposit molybdenum oxide on the powder-type titanium dioxide, in order to achieve a high level of effectiveness. The pre-calcined material is then mixed again with water and kneaded into a viscous mass. The mass is shaped into the desired structures and then calcined.